[gdal-dev] Datum transformation method when projecting from a sphere to NAD83 shapefile

[Frank Warmerdam]

Limei Ran wrote:
> Hi:
> 
> I am using ogr2ogr to project a shapefile defined in a sphere to NAD83.  
> My command is like the following:
> 
> ogr2ogr -t_srs "+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 
> +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs " -s_srs 
> "+proj=lcc +a=6370000.0 +b=6370000.0 +lat_1=33 +lat_2=45 +lat_0=40 
> +lon_0=-97" wrf12km_nc_g152.shp wrf12km_nc.shp
> 

Limei,

You have not provided a datum definition for the source coordinate
system, only an ellipsoid definition.

If you are using PROJ 4.6.0 or newer no datum transformation at all
will be applied.  The LCC coordinates will be reprojected to lat/long
on the indicated ellipsoid, and then those lat/long values will
projected into AEA on the NAD83 datum with no attempt to do any
"change of ellipsoid" or datum shift between the LCC and AEA geographic
coordinate system.

If you are using anything older than PROJ 4.6.0, there will be a
change of ellipsoid applied (which will be quite substantial since the
LCC is on a sphere) implemented by converting from lat/long on the
LCC sphere, to geocentric coordinates, and then from geocentric coordinates
to lat/long on the AEA ellipsoid.  This will result in large changes in
latitude.

So, basically, the approach used depends on the software version.

Best regards,
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